1. Field of the Invention
The present invention relates to a prothesis for use in the middle ear, and more particularly, to a prothesis for holding a magnet for use with an electromagnetic induction hearing aid.
2. Description of the Related Art
One common problem with in-the-ear acoustic hearing aids has been low fidelity. This low fidelity results from the very small size of the receiver or speaker, standing waves created in the ear canal and feedback because of the venting of the hearing aid. An alternative to resolve this problem has been the use of an electromagnetic induction hearing aid, wherein a magnet is coupled to the ossicles of the middle ear, the tympanic membrane or the cochlea and driven into motion by an electromagnetic field produced by an output coil in the hearing aid. This concept was first proposed by Rutschmann in an article entitled "Magnetic Audition--Auditory Stimulation by means of Alternating Magnetic Fields Acting on a Permanent Magnet Fixed to the Eardrum," IRE Transactions on Medical Electronics, Mar. 1959, pp. 22-23. Further work indicating promise was done by Richard L. Goode as indicated in his two articles "An Implantable Hearing Aid," Tr. Am. Acad. Ophthalmol & Otholaryngol, 74:128-139, Jan.-Feb. 1970 and "Audition via Electromagnetic Induction," Arch Otolarnygol, 98:23-26, Jul. 1973. Goode in most of his tests glued a magnet to the tympanic membrane which was then vibrated by either a coil located behind the ear or a special, externally driven coil inserted in the ear canal. In other tests Goode temporarily affixed the magnet to various locations in the middle ear and affixed the magnet to a Silverstein malleus clip. Goode indicated the feasibility and desire to develop such as system. A similar system was shown in Belgium U.S. Pat. No. 833,809, which was published on Mar. 25, 1976.
The field lay basically dormant for a number of years until work was commenced by Drs. Jack Hough, Jack Vernon and Kenneth Dormer and Messrs. Jorgen Heide, Anthony Prescott and Timothy Gooch. The work of these two groups resulted in a series of U.S. Pat. Nos., 4,606,329, 4,776,322 and 4,800,884. These patents show various behind the ear and in the ear versions of hearing aids which have coils which develop magnetic fields which are coupled to magnets located in the middle ear or coupled to the ossicles.
One problem with electromagnetic induction has been that magnet materials are basically not bio-compatible, especially the magnet materials having sufficient energy densities so that higher efficiency and therefore longer battery life can be obtained. To this end some method or means of bio-compatibility has been required. Hough, et al. indicated in the various patents that the magnet material could be impregnated into a bio-compatible material and formed into a prosthesis having various shapes as shown in the patents. Other alternatives for magnet placement were shown in Heide Pat. No. 4,800,884. One embodiment was the use of a magnet located at the end of a Silverstein malleus clip, with the clip passing through the tympanic membrane and being coupled to the malleus. In a second embodiment a biocompatibly coated magnet was located between the tympanic membrane and the malleus. Various other designs have been developed to couple the magnet to the ossicles. For example, in U.S. Pat. No. 4,817,607 to Tatge, the magnet is located in the head of a partial or total ossicular replacement prosthesis. Alternatively, in U.S. Pat. No. 4,840,178 to Heide et al. the magnet is formed in two pieces which are hinged together and clasp around the long process of the incus. Yet another variation is shown in U.S. Pat. application Ser. No. 702,396 to Heide et al., where the magnet is removably attached to a transtympanic rod which passes through the tympanic membrane and is affixed to the malleus either by means of a snug fit into a hole drilled in the malleus or a clamp around the malleus.
While there are thus numerous variations shown for coupling the magnet to the ossicles, there are disadvantages in several cases and other cases which are not adequately covered by the known protheses. For example, in many cases the patient has a fully functional ossicular chain and therefore the use of a replacement prothesis as shown Tatge or Hough is not desirable. Further, the magnets and prothesis as shown in Heide '178 and Hough are quite difficult to construct because of the unusual shapes of the magnetic materials. Other variations pass through the tympanic membrane, which while reducing biocompatibility concerns, does have the problem that the tympanic membrane has been traversed and this may be undesirable for a long term installation in some cases.
Therefore it is desirable to have a prosthesis which allows operation of a fully intact ossicular chain, does not require convoluted and complicated magnet shapes and is wholly contained in the middle ear cavity.